• Accepted Paper

Mapping the stability of spin qubits in superconducting pseudogap systems

Chen-How Huang and Miguel A. Cazalilla

Phys. Rev. B - Accepted 25 September, 2026

DOI: https://doi.org/10.1103/c7l8-g843

Abstract

Superconducting spin qubits, realized as Yu-Shiba-Rusinov spin-doublet states in quantum-dot-superconductor systems, represent a cornerstone of current research in quantum technologies. We analyze these ground states of quantum impurities in superconducting pseudogap systems, namely systems with a pseudogap tunneling density of states ρ(ϵ)∼|ϵ|r for energies |ϵ|≫Δ (Δ being a s-wave pairing potential). For r=1, these hosts are realized as Dirac materials (graphene or 3D topological insulator surfaces) in proximity to conventional superconductors, or as d+is superconductors. Using effective field theory and numerical renormalization group, we map the phase diagram against the pseudogap exponent r>0 and particle-hole symmetry-breaking perturbations. At particle-hole symmetry, increasing r also increases the critical value, Jc, of the Kondo coupling that triggers the transition from spin doublet to singlet. Unlike the gapless pseudogap Kondo systems, numerical and analytical evidence suggest that Andreev reflection stabilizes a singlet ground state at J for all r>0. Breaking particle-hole symmetry —by potential scattering or chemical potential— eventually restores the transition at lower Jc. Our results indicate that coupling to superconducting hosts with large pseudogap exponents enhances the stability of spin qubits at large Kondo coupling.

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